1. Field of the Invention
The present invention relates to a method for improving the flex crack resistance, cut resistance and tear strength of polyurethane products.
2. Description of the Prior Art
Polyurethane has high strength, abrasion resistance and weather resistance, and the use of polyurethane has remarkably increased in recent years. Particularly, casting type polyurethane is widely used due to its processability in the liquid state. However, polyurethane is insufficient in flex crack resistance, cut resistance and tear strength, and when a polyurethane product is once flawed, the flaw rapidly grows up under a severe use condition to give a fatal blow to the product. In order to obviate the above described drawbacks of polyurethane, the following methods have hitherto been proposed, i.e., a method wherein a plasticizer is added to a polyurethane prepolymer, and a method wherein a curative (chain extender) is added to a polyurethane prepolymer in an amount of not less than equivalent based on the amount of isocyanate groups in the prepolymer. However, in the former method which uses a plasticizer, the curing and chain extending reactions of the polyurethane prepolymer are often adversely affected, and further the physical properties of the resulting polyurethane always considerably decrease. The latter method, wherein polyamine or polyol is added to a polyurethane prepolymer in an amount of not less than chemical equivalent based on the amount of isocyanate groups of the prepolymer to effect the curing and chain extension of the prepolymer, has been known for a long time as a method of obtaining a polyurethane having an improved flex crack resistance. However, polyamines and polyols, which give a high strength to polyurethane, are expensive. Particularly, polyamines have problems in toxicity to human body, particularly in carcinogenecity. Moreover, the use of a large amount of such chain extenders shortens the pot life at the casting, and often causes difficulties in the casting operation. Accordingly, it is desirable to decrease the amount of chain extender as much as possible. This fact is clearly contrary to the commercial demand for obtaining polyurethane having high strength, flex crack resistance, cut resistance and tear strength.